An EKG (electrocardiogram) shows the electrical activity of your heart, recorded as a series of waves on paper or a screen. Each wave corresponds to a specific event: the upper chambers contracting, the lower chambers contracting, and the heart resetting for the next beat. From this 10-second snapshot, a doctor can identify irregular rhythms, signs of a current or past heart attack, thickened heart walls, and even dangerously abnormal potassium levels.
How an EKG Records Your Heart
A standard 12-lead EKG uses 10 small adhesive electrodes placed on your chest, arms, and legs. Six go across the left side of your chest in precise positions between your ribs and along your side. The remaining four go on your wrists (or shoulders) and near your ankles. Despite having only 10 physical electrodes, the machine calculates 12 different “views” of your heart’s electrical signal by comparing readings between electrode pairs. The entire recording takes about 10 seconds and is completely painless.
The result is a strip of squiggly lines printed on grid paper. Each small square on that paper represents a fixed amount of time (0.04 seconds) and a fixed amount of voltage. That grid is what makes EKG readings so precise: doctors can measure intervals down to the millisecond and spot tiny voltage changes that signal trouble.
What Each Wave Means
The EKG tracing has three main landmarks, and each one maps to a physical event in your heart.
The P wave is the first small bump. It represents the electrical signal spreading across the two upper chambers (atria), causing them to squeeze and push blood into the lower chambers. A normal P wave is smooth and small. If it’s too wide, too tall, or missing entirely, that points to problems in the upper chambers.
The QRS complex is the tall, sharp spike that follows. It represents the electrical signal firing through the two lower chambers (ventricles), which do the heavy lifting of pumping blood to your lungs and body. This complex is much larger than the P wave because the ventricles are bigger and generate more electrical force. A normal QRS lasts less than 0.12 seconds. When it’s wider than that, it can mean the electrical signal is traveling through the ventricles on an abnormal path.
The T wave is the rounded bump after the QRS. It represents the ventricles resetting their electrical charge to prepare for the next heartbeat. Changes in the T wave’s shape, height, or direction are some of the most important clues doctors look for, because they can signal everything from a heart attack in progress to a potassium imbalance.
Heart Rate and Rhythm
The most basic thing an EKG shows is how fast and how regularly your heart beats. Doctors can calculate your heart rate by counting the number of large grid squares between two consecutive QRS spikes and dividing that number into 300. Three large squares between beats means a rate of 100 beats per minute. Five large squares means 60.
Rhythm is equally important. In a healthy heart, the spacing between beats is consistent and every QRS spike is preceded by one P wave. When the rhythm is irregular or P waves are absent, that points to an arrhythmia. Atrial fibrillation, the most common serious rhythm disorder, shows up as a completely irregular pattern with no identifiable P waves, replaced instead by chaotic electrical activity in the upper chambers.
Signs of a Heart Attack
One of the most critical uses of an EKG is detecting a heart attack, either happening right now or one that occurred in the past.
During an active heart attack, the segment of the tracing between the QRS spike and the T wave (called the ST segment) shifts away from its normal baseline. When this segment rises significantly above the baseline, it signals a complete blockage of a coronary artery, a condition known as a STEMI. This is the type of heart attack that requires emergency treatment to reopen the artery. When the ST segment drops below baseline instead, or when the T waves flip upside down, that pattern suggests a partial blockage, classified as an NSTEMI.
A past heart attack leaves a different fingerprint. Dead heart tissue can’t conduct electricity, so the EKG develops abnormally deep or wide “Q waves” at the beginning of the QRS complex. Historically, a Q wave lasting 0.04 seconds or deeper than 25% of the following spike has been considered a marker of old damage, a criterion that has been in use since 1934. These Q waves can persist for years or even permanently, giving doctors a record of previous injury even if you didn’t realize you had a heart attack.
Structural Changes in the Heart
When the heart walls thicken or chambers enlarge over time (from chronic high blood pressure, valve disease, or other conditions), the EKG reflects those changes through voltage. A thicker ventricle generates a stronger electrical signal, producing taller-than-normal QRS spikes. A thicker or enlarged atrium produces wider or taller P waves. These voltage patterns aren’t as precise as an ultrasound for measuring wall thickness, but they’re often the first clue that something structural is going on.
Potassium Imbalances
Your heart cells rely on potassium to fire and reset properly, so abnormal potassium levels produce distinctive EKG changes, sometimes before you feel any symptoms at all.
High potassium (above 5 mmol/L) makes T waves tall and sharply peaked. As levels climb further, the P wave flattens or disappears, the QRS complex widens, and the interval between beats stretches out. In severe cases, the T wave and QRS can merge into a smooth, sinuous “sine wave” pattern that signals a life-threatening situation. About half of patients with critically high potassium show these telltale changes on their EKG.
Low potassium (below 3.5 mmol/L) produces the opposite pattern. T waves become shallow or flat, and a new bump called a U wave appears after the T wave. When potassium drops below 3 mEq/L, the U wave can grow so large it swallows the T wave entirely, making the overall tracing look deceptively like a prolonged heartbeat interval.
What a Normal EKG Does Not Rule Out
A normal EKG is reassuring, but it has real blind spots. The test captures only 10 seconds of your heart’s activity, so intermittent problems like occasional arrhythmias can easily be missed. More importantly, a resting EKG is not very good at detecting blocked arteries that haven’t yet caused damage. In one study comparing EKGs to coronary angiography (the gold-standard imaging for artery blockages), the EKG correctly identified significant blockages only about 52% of the time. That means nearly half of patients with meaningful coronary artery disease had a normal-looking resting EKG.
This is why doctors often order additional tests, like a stress test (EKG recorded while you exercise) or imaging studies, when they suspect coronary artery disease despite a normal resting tracing.
Smartwatch EKGs vs. Clinical EKGs
Consumer devices like the Apple Watch can now record a single-lead EKG from your wrist. These are genuinely useful for one specific purpose: catching atrial fibrillation. In clinical testing, the Apple Watch detected atrial fibrillation with 100% sensitivity and over 99% specificity, meaning it rarely misses the condition and rarely flags it falsely.
For other abnormalities, performance drops. The watch detected premature ventricular contractions (extra beats originating in the lower chambers) only 55% of the time, and wide QRS complexes about 77% of the time. Overall, its sensitivity for picking up any abnormal EKG was around 88%, with specificity near 93%. A single-lead recording simply can’t replicate the 12 different angles a clinical EKG provides, so it works best as a screening tool rather than a diagnostic one.